Unbelieveably small, cramped, no air places don't really bother me but I wouldn't get in that thing. There's no way you could expect someone to give a good vocal performance inside something that small. I think all you really need to do is build some of Ethan Winners diffuser panels, mount them on mic stands or whatever and circle them around the vocalist. A lot cheaper, easier to build and portable. Hang them on your walls for diffusion during mixdown.
DD